Myelin, the multilayered membrane sheath that surrounds axons, facilitates the rapid conduction of nerve impulses. Oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS) are responsible for the formation of the myelin sheath. These cells express a number of proteins unique to myelin that are required for the formation and maintenance of this highly organized structure. Over the past decade, the characterization of genes that encode myelin proteins, and of the phenotype of mouse mutants defective in these genes, has greatly increased our understanding of the myelination process. Nevertheless, although much has been learned concerning the structural myelin proteins, little is known regarding the regulation of the genes that encode the enzymes responsible for the synthesis of the lipids of myelin. We have recently isolated the cDNA of a gene that encodes the enzyme UDP galactose:ceramide galactosyltransferase (CGT), which is responsible for catalyzing the final step in the synthesis of galactocerebroside (GalC), the predominant glycolipid of myelin. Recent evidence suggests that GalC has important regulatory and structural functions in the myelination process. Experiments are proposed that are designed to gain a better understanding of the functional properties of CGT, as well as the gene that encodes this enzyme. Furthermore, mouse mutants in the CGT gene will be generated using the gene targeting approach in embryonic stem cells. These mice should provide us with an animal model system with which to examine the importance of GalC in the myelination process. The availability of these reagents should allow for the further characterization of the molecular details of myelin lipid synthesis.